The present invention relates to a mechanism for converting rotary motion into linear motion, in which a kinetic force is converted between rotary motion and linear motion, and more particularly, to a mechanism for converting rotary motion into linear motion, which is high in conversion efficiency and suited to a power steering device.
In recent years, power steering devices constitute an accessory essential to automobiles. In these power steering devices, in place of conventional hydraulic assist type systems, electrically-driven assist type systems have occupied the main stream in recent years to contribute to energy saving.
By the way, with the electrically-driven assist type systems, it is general to use an electric motor for an assist power source. In the case where a steering device, to which an electric motor is applied, is of a rack and pinion system, linear drive forces are necessary, so that a mechanism that converts rotary motion into linear motion, that is, a so-called mechanism for converting rotary motion into linear motion is used.
Besides, in this case, since it is desirable from the miniaturization point of view to use an electric motor having a high rotating speed, a mechanism uniting with a speed reducer is demanded as a mechanism for converting rotary motion into linear motion, and thus, for example, a ball-screw type mechanism for converting rotary motion into linear motion has been conventionally proposed (for example, see JP-A-7-165049).
With the mechanism or device thus proposed, a threaded rod is connected integrally to a rack in a rack and pinion type steering device, a nut meshes with the rod, and the nut is rotated by an electric motor, which constitutes a rotary power source, to thereby cause the rack to make translation (linear movement).
In this case, since the rack is moved an amount corresponding to a lead of the thread when the electric motor is caused to make one revolution, a large reduction ratio is obtained by decreasing a lead angle whereby the electric motor is increased in rotating speed to achieve miniaturization.
Since a large load acts between the thread of the rod and the thread of the nut, a multiplicity of balls are arranged there and circulated to make rolling contact, thus reducing friction to attain high efficiency.
In the related art, however, means for circulation of the multiplicity of balls is essential, and when circulation of the balls is not smooth, slide friction is generated between the balls and the nut and between the balls and the rack whereby the balls are increased in coefficient of friction to lead to reduction in conversion efficiency.
In particular, in order to make a motor small in size, threads must be made small in lead angle (around 5 degrees in the existing state) in a steering device, which is set to be large in reduction ratio, so that a remarkable decrease in efficiency is resulted as shown in FIG. 10 when balls are increased in coefficient of friction (around 0.01 in the existing state).
FIG. 10 shows the relationship between a lead angle and efficiency of a ball screw mechanism with a coefficient of friction of balls as a parameter. As shown in the figure, it is found that as the coefficient of friction of balls increases from around 0.01, the conversion efficiency decreases.
With the related art, once slippage begins to generate, rolling surfaces of balls begin to roughen, which brings about further slippage to cause a catastrophic, rapid rise in coefficient of friction of balls, thus giving rise to a fear of breakage of the mechanism in a short time.
Therefore, it is a supreme task in such mechanism to maintain a state of circulation of balls favorable at all times, so that high accuracy of balls, nut threads, and rack threads in shape and dimensions is a most important point as well as optimum design of a ball return path, which leads to an increase in cost.
Since a necessary accuracy is rapidly heightened as the balls are increased in number, an actual limit is determined on an upper limit of the number of balls in terms of cost while the number of balls determines a maximum output that can be generated by the mechanism.
Accordingly, with a ball screw mechanism according to the related art, a practically upper limit comes out in transmission force, so that power steering devices making use of the mechanism involve a problem that they cannot be mounted on large-sized cars, of which a large output (rack thrust) is demanded.